[Paper Review] Low frequency noise in ballistic Nb/Ag point contacts

Jian Wei, Goutam Sheet+1 more|arXiv (Cornell University)|Apr 30, 2010

Physics of Superconductivity and Magnetism1 references4 citations

TL;DR

This study investigates low-frequency noise in ballistic Nb/Ag point contacts using Andreev reflection spectroscopy to confirm ballistic transport and analyze noise characteristics. It identifies that two-level fluctuators (TLFs) dominate at high bias, with their dynamics linked to superconducting order parameter depairing, particularly in contacts requiring a large depairing parameter Γ in BTK fitting, suggesting TLFs may explain anomalous Γ values in conventional superconductors.

ABSTRACT

We report on the low frequency noise in the ballistic point-contacts between a silver tip and a niobium foil. The ballistic nature of the point-contacts is confirmed by Andreev reflection spectroscopy at low bias voltage with the Nb foil cooled below its superconducting transition temperature ($T_c$). We find that the voltage dependence of the low frequency noise differs for different point-contacts with varying contact resistances. At high bias voltages, random two level fluctuations appear and dominate over the background $1/f$ noise. From analysis of the Andreev reflection spectra, we show that such two level fluctuators may give rise to depairing of the superconducting order parameter.

Motivation & Objective

• To investigate low-frequency voltage noise in ballistic Nb/Ag point contacts formed on superconducting Nb foils.
• To determine whether two-level fluctuators (TLFs) dominate noise at high bias and how they affect transport properties.
• To examine the origin of anomalously high depairing parameters (Γ) in BTK fitting of Andreev reflection spectra.
• To assess whether TLFs could be responsible for apparent deviations from standard BCS behavior in conventional superconductors.
• To evaluate the role of TLFs in modifying the superconducting gap and resistance dynamics in nanoscale point contacts.

Proposed method

• Ballistic point contacts were formed between a silver tip and a niobium foil using a low-temperature scanning probe microscope at 5.8 K.
• Andreev reflection spectroscopy (PCAR) was used to confirm ballistic transport and extract BTK fitting parameters, including resistance (R_N), gap (Δ), barrier strength (Z), and depairing parameter (Γ).
• Low-frequency voltage noise was measured using a DC bias with a 3 kΩ series resistor, AC-coupled to battery-powered low-noise amplifiers.
• Noise power spectra were analyzed in the 1–100 kHz range, with normalization to fS_V/V² to assess 1/f noise scaling and identify TLF contributions via Lorentzian fits.
• The effective time constant (τ_eff) of TLFs was extracted using τ_eff = τ₀exp(ε/V), with τ₀ and ε fitted from spectral knee positions.
• Measurements were repeated at different temperatures (5.8 K, 13 K, 23 K) to assess thermal stability and temperature dependence of TLFs.

Experimental results

Research questions

• RQ1 Do two-level fluctuators (TLFs) dominate low-frequency noise in ballistic Nb/Ag point contacts at high bias voltages?
• RQ2 Can the observed TLF behavior explain anomalously high depairing parameters (Γ) in BTK fitting of Andreev reflection spectra?
• RQ3 How do TLF dynamics correlate with the superconducting gap and resistance in nanoscale point contacts?
• RQ4 Is the presence of TLFs responsible for apparent deviations from standard BCS theory in conventional superconducting point contacts?
• RQ5 How does temperature affect the stability and spectral characteristics of TLFs in ballistic Nb/Ag junctions?

Key findings

• Two-level fluctuators (TLFs) dominate the noise spectrum at high bias voltages (e.g., 20–30 mV) in point contact A, with Lorentzian-shaped power spectra and a frequency knee at ~100 Hz.
• The TLF effective time constant τ_eff follows τ_eff = τ₀exp(ε/V), with τ₀ ≈ 10⁻¹² s and ε ≈ 483 mV, consistent with defect heating models.
• For point contact F, which required a large depairing parameter (Γ = 0.5 mV) in BTK fitting, TLFs strongly dominate the noise across the entire bias range, with large spectral weight at 10²–10³ Hz.
• The TLF behavior in contact F is thermally sensitive, with peak positions and magnitudes changing significantly upon heating, indicating easy thermal activation of fluctuators.
• The absence of correlation between TLF positions and phonon peaks (from d²V/dI²) rules out phonons as the origin of TLFs, suggesting atomic-scale cluster rearrangements at the interface.
• The study proposes that TLFs may be the underlying cause of anomalously high Γ values in BTK fitting, challenging the assumption that such values reflect intrinsic superconducting properties.